Differential display of mRNA has been used to identify genes whose expression may drive or prevent progression to tumor cell phenotype. One such gene, tissue inhibitor of metalloproteinases TIMP-3), was expressed in preneoplastic but not neoplastic JB6 cells (Sun et al., Cancer Res, 1994; Sun et al., J Biol Chem, 1995) and was found to act as a tumor suppressor when expressed in human DLD colon carcinoma cells (Bian et al., Carcinogenesis, 1996). We are thus interested in the molecular basis for transcriptional repression of TIMP-3(Kim et al.,Bioch. J, 1997 and Pennie et al., Cell Growth and Diff, 1999). The TIMP-3 promoter is hypermethylated in neoplastic JB6 cells and the methylase inhibitor 5-azacytidine upregulates TIMP-3 expression. PCR based mapping revealed that three of the HpaII methylation sites show differential methylation in neoplastic and preneoplastic JB6 cells, suggesting the importance of sequence specific methylation in regulating TIMP-3 transcription. Expression of antisense DNA methyl transferase restored expression of TIMP-3 to tumor cells and recapitulated the unmethylated status of the same sites associated with TIMP-3 expression in preneoplastic JB6 cells (Pennie et al, Cell Growth and Diff, 1999). A separate differential display analysis was carried out to identify gene expression that may mediate or inhibit tumor promoter dependent stages of progression. Full length cDNA clones have been isolated for two genes preferentially expressed in promotion resistant mouse JB6 cells. One designated pdcd4 (Cmarik et al., PNAS 1999, is novel; the other is plekstrin (Cmarik et al Genomics 2000). Antisense expression of the novel pdcd4 gene converts P- to P+ cells and pdcd4 sense expression converts P+ to P- cells, thus establishing a causal relationship to prevention of tumor promoter induced transformation. Current studies focus on the molecular function and localization of pdcd4 protein. Examination of the possible inhibitory effect of pdcd4 on molecular events known to be required for tumor promotion revealed that pdcd4 expression inhibited the activation of transcription factor AP-1 but not of NFkappa B or of ornithine decarboxylase( Yang et al submitted). Discovery of pdcd4 binding partners by yeast two-hybrid analysis is expected to clarify the molecular mechanism by which pdcd4 prevents cancer. Finally, we have discovered that expression of the chromatin protein(s)HMG I(Y)is preferentially induced by tumor promoters in promotion sensitive cells (Cmarik et al, Oncogene, 1998), and are investigating the causal significance of this as well as the possible interaction of HMG I(Y) with other molecules whose activation is required for transformation.